A voltage regulator can be designed either as a positive voltage regulator or a negative voltage regulator. For convenience, this invention will be described as it relates to a positive voltage regulator, although it will be clear to one skilled in the a how to apply the invention to a negative regulator by appropriate reversal of voltage polarities and use of complementary transistor types.
In a positive voltage regulator, the input voltage V IN must be larger than the desired output voltage V OUT, by an increment known as the "dropout voltage." If V IN is too low, the regulator will be unable to hold V OUT to the desired level. If V IN should then fall, V OUT must fall as well. A low dropout voltage is important, for example, in battery powered equipment where it is desirable to maintain V OUT at its designed level for as long as possible as the battery voltage falls. In today's low dropout voltage regulators, the dropout voltage can be as low as 500 millivolts.
Heretofore, the zener diode has been the primary component of a voltage regulating circuit. Zeners are supplied for a quoted voltage, which is always defined at a given resistance current Iz. At this current, it will be within the specified tolerance, but at other currents it will differ, the difference being a function of the zener slope resistance Rs. Over some range of Iz, Rs can be assumed to be fairly linear. As the current decreases, however, the characteristic approaches the "knee" of the curve and Rs increases sharply. There is very little point in operating a zener intentionally on the knee. The actual knee current depends on the type and voltage but is rarely less than a few hundred .mu.A. In fact, zener diodes are typically rated for currents in milliamps--most are rated for currents of 3 mA to 250 mA. Consequently, zeners are not much use for micropower circuits.
In certain line powered telephone devices, in which the voltage is typically regulated to 3 volts, the current level may be as low as 10 to 25 .mu.A, or even less. The commercially available zener diode closest to meeting this requirement is rated for 2.7 V at 50 .mu.A. As will be readily appreciated by those skilled in the art, it is extremely difficult and costly to design a voltage regulator circuit where the reference voltage is so close to the regulating voltage.